ST. PAUL, MN -- People who spent more time in the sun as children may have a lower risk of developing multiple sclerosis (MS) than people who had less sun exposure during childhood, according to a study published in the July 24, 2007, issue of Neurology, the medical journal of the American Academy of Neurology.

For the study, researchers surveyed 79 pairs of identical twins with the same genetic risk for MS in which only one twin had MS. The twins were asked to specify whether they or their twin spent more time outdoors during hot days, cold days, and summer, and which one spent more time sun tanning, going to the beach and playing team sports as a child.

The study found the twin with MS spent less time in the sun as a child than the twin who did not have MS. Depending on the activity, the twin who spent more hours outdoors had a 25 to 57 percent reduced risk of developing MS. For example, the risk of developing MS was 49 percent lower for twins who spent more time sun tanning than their siblings.

Sun exposure appears to have a protective effect against MS, said study authors Talat Islam, MBBS, PhD, and Thomas Mack, MD, MPH, with the Keck School of Medicine of the University of Southern California in Los Angeles. Exposure to ultra violet rays may induce protection against MS by alternative mechanisms, either directly by altering the cellular immune response or indirectly by producing immunoactive vitamin D.

The study also found the protective effect of sun exposure was seen only among female twin pairs, but Mack says this novel finding must be viewed with caution since only a few male twins were involved in the study.

Our findings note the importance of sun exposure among people with identical genetic risk for MS, said Mack. High priority should be given to research into how sun exposure reduces MS risk if we are to unravel the mystery of what causes MS.